Method and system for testing a sonet network element

ABSTRACT

Method and system are described for testing SONET network elements (NE) against various SONET logical layer criteria or standards. A &#34;basic&#34; signal is developed or generated to meet many of the criteria in order to minimize the number of alarms or abnormal conditions reported by the NE. Simple procedures are provided to modify the basic signal. The test signals are utilized to test a network element&#39;s ability to detect and respond to incoming signal failures and maintenance signals, its payload mappings, and its use of various overload bits and bytes.

TECHNICAL FIELD

This invention relates to testing methods and systems and, inparticular, to testing methods and systems for SONET network elements.

BACKGROUND ART

A synchronous optical network (SONET) is well known in the art andincludes network elements (NEs) such as Add-Drop Multiplexers (ADM's).With respect to operations data networking, the SONET NEs are eitherGateway NEs, Intermediate NEs, or End NEs as illustrated in FIG. 1.

SONET signals have rates and formats as defined in BELLCORE TECHNICALREFERENCE entitled "Synchronous Optical Network (SONET) TransportSystems: Common Generic Criteria", TR-NWT-00253 (TR 253), which containsmany criteria that are applicable to SONET NEs. This is realized bydefining a basic signal of 51.840 Mb/s and a byte interleaved multiplexscheme. The basic signal can be divided into a portion assigned tooverhead and a portion that carries the payload. Payload pointers aremechanisms that allow the payload to shift relative to the overhead,thus permitting the accommodation of different signal phases and framerates in multiplexing.

The Synchronous Transport Signal-level 1 (STS1) is the basic modularsignal. Its rate is 51.840 Mb/s. The optical counterpart of the STS-1 isthe Optical Carrier-level 1 signal (OC-1), which is the result of adirect optical conversion of the STS-1 h after frame synchronousscrambling.

The definitions of the first levels (STS-1 and OC-1) define the entirehierarchy of synchronous optical signals because the higher levelsignals are obtained by synchronously multiplexing lower level signals.The higher level signals are denoted by STS--N and OC--N, where N is aninteger. There is an integer multiple relationship between the rates ofthe basic module OC-1 and the multiplexed signal OC--N (i.e., the rateof OC--N is equal to N times the rate of OC-1).

SONET optical transmission systems support only certain values of N.Currently, these values are 1, 3, 12, 24 and 48. Table I lists standardoptical carrier rates from 51.840 Mb/s up through 2488.320 Mb/s.

                  TABLE I                                                         ______________________________________                                        Line Rates for The Allowable OC-N Signals                                     OC Level      Line Rate (Mb/s)                                                ______________________________________                                        OC-1          51.840                                                          OC-3          155.520                                                         OC-12         622.080                                                         OC-24         1244.160                                                        OC-48         2488.320                                                        ______________________________________                                    

The SONET transport format presented here is based on ANSI T1.105. FIG.2 depicts the STS-1 synchronous payload envelope (SPE). The STS--Nsignal is formed by byte innerleaving N STS-1 signals. The VirtualTributory (VT) structure is designed for transport and switching ofsub-STS-1 payloads. There are four sizes of VTx: VT 1.5 (1.728 Mb/s),VT2 (2.304 Mb/s), VT3 (3.456 Mb/s), and VT6 (6.912 Mb/s). In the 9-rowstructure of the STS-1 SPE, these VTs occupy 3 columns, 4 columns, 6columns, and 12 columns, respectively.

The overhead and transport functions are broken into layers thatincrease in complexity from the viewpoints of hardware and the opticalinterface frame format. The layers are Physical, Section, Line and Path,as illustrated in FIGS. 3 and 4. The layers have a hierarchicalrelationship and are considered from the top down. The top-down approachprovides a general introduction to the individual layers and theirfunctionalities.

Each layer requires the services of all lower level layers to performits function as illustrated in FIG. 4. For example, suppose that twoPath layer processes are exchanging DS3s. The DS3 signal and STS POH aremapped into an STS-1 SPE, which is then given to the Line layer. TheLine layer multiplexes several inputs from the Path layer (frame andfrequency aligning each one) and adds Line overhead (e.g. overheadrequired for protection switching). Finally, the Section layer providesframing and scrambling before optical transmission by the Physicallayer.

SONET Interface Layers

This section describes each layer in detail. Each description includes abroad classification of the layer, followed by a specification of themain functions it provides. Finally, examples of system hardwareassociated with the layer are given to clarify the role it plays. FIG. 4depicts the relationship of the layers to each other.

Physical Layer

The physical layer deals with the transport of bits as optical orelectrical pulses across the physical medium. No overhead is associatedwith the physical layer.

Section Layer

The section layer deals with the transport of an STS--N frame across thephysical medium. This layer uses the physical layer for transport.

Functions of this layer include framing, scrambling, section errormonitoring, and communicating section level overhead (such as LocalOrderwire (LOW)). The overhead defined for this layer is interpreted andmodified or created by Section Terminating Equipment (STE).

The section and physical layers can be used in some equipment (e.g. theSTE regenerator) without involving the higher layers.

Line Layer

The line layer deals with the transport of STS SPE path layer payloadand its overhead across the physical medium. All lower layers exist toprovide transport for this layer.

This layer provides synchronization and multiplexing functions for thepath layer. The overhead associated with these functions includesoverhead for maintenance and protection purposes and is inserted intothe line overhead channels. The line overhead for this layer isinterpreted and modified or created by Line Terminating Equipment (LTE).Because the LTE contains section layer functions, it is also an STE.

An example of system equipment that communicates at this level is anOC--N to OC--M multiplex.

Path Layer

The path layer deals with the transport of network services betweenSONET terminal multiplexing equipment. Examples of such services areDS1s and DS3s.

The path layer maps the services into the format required by the linelayer. In addition, this layer communicates end-to-end via the POH. Theoverhead defined for this layer is interpreted and modified or createdby Path Terminating Equipment (PTE). Because the PTE contains line andsection layer functions, it is also considered an LTE and STE.

An example of system equipment that communicates at this level is DS3 toSTS-1 mapping circuits.

Interaction of the Layers

FIG. 4 depicts the interaction of the optical interface layers. Eachlayer

• communicates horizontally to peer equipment in that layer;

• processes certain information and passes it vertically to the adjacentlayers.

The interactions are described in terms of each level's horizontal andvertical transactions.

FIG. 4 also shows network services as inputs to the path layer. Thislayer transmits horizontally to its peer entities the services and thePOH. The path layer maps the services and POH into SPEs that it passesvertically to the line layer.

The line layer transmits to its peer entities SPEs and the line layeroverhead. It maps the SPEs and line overhead into STS--N frames. SPEsare synchronized and multiplexed at this time and then the STS--N signalis passed to the section layer.

The section layer transmits to its peer entities STS--N signals withsection layer overhead (e.g. LOW). It maps STS--N and the sectionoverhead into bits that are handed to the physical layer, whichtransmits optical or electrical pulses to its peer entities.

Access to all four layers is not required of every SONET NE. Forexample, an STE regenerator would use only the first two layers(physical and section). Also, an NE that merely routes SPEs and does notaccept any new inputs from the path layer only uses the first threelayers (physical, section and line). However, these NEs may monitor theoverhead of layers that they do not terminate.

STS-1 Payload Pointers

The STS-1 Payload Pointer provides a method of allowing flexible anddynamic alignment of the STS SPE within the STS Envelope Capacity,independent of the actual contents of the envelope.

Dynamic alignment means that the STS SPE is allowed to float within theSTS Envelope Capacity. Thus, the pointer is able to accommodatedifferences not only in the phases of the STS SPE and the TransportOverhead, but in the frame rates as well.

Pointer Value

The Payload Pointer contained in H1 and H2 of the Line Overheaddesignates the location of the byte where the STS SPE begins. The twobytes allocated to the pointer function can be viewed as one word, asFIG. 5 shows. Bits 7 through 16 of the pointer word carry the pointervalue. Bits 5 and 6 of the pointer word are set to zero except whentransmitting STS Path AIS.

As FIG. 6 illustrates, the pointer value is a binary number with a rangeof 0 to 782, which indicates the offset between the pointer and thefirst byte of the STS SPE. The Transport Overhead bytes are not countedin the offset. For example, a pointer value of 0 indicates that the STSSPE starts in the byte location that immediately follows the H3 byte(pointer action byte), whereas an offset of 87 indicates that the STSSPE starts immediately after the K2 byte.

Frequency Justification

When there is a frequency offset between the frame rate of the TransportOverhead and that of the STS SPE, then the pointer value is incrementedor decremented as needed, accompanied by a corresponding positive ornegative stuff byte.

When the frame rate of the STS SPE is less than that of the TransportOverhead, the alignment of the envelope periodically slips back in timeand the pointer is incremented by one. This operation is indicated byinverting bits 7, 9, 11, 13 and 15 (I-bits) of the pointer word. Apositive stuff byte appears immediately after the H3 byte in the framecontaining inverted I-bits. Subsequent pointers contain the new offset.FIG. 7 illustrates the positive pointer justification.

When the frame rate of the STS SPE is greater than that of the TransportOverhead, the alignment of the envelope is periodically advanced in timeand the pointer is decremented by one. This operation is indicated byinverting bits 8, 10, 12, 14 and 16 (D-bits) of the pointer word. The H3byte becomes a negative "stuff" byte, i.e. carries an SPE byte in theframe containing inverted D-bits. Subsequent pointers contain the newoffset. FIG. 8 illustrates the negative pointer justification.

In the criteria of TR 253, it is an objective that theincrement/decrement decision be made at the receiver by a match of 8 ormore of the 10 I- and D-bits to either the increment or decrementindication. This method is an extension of the majority vote method forI- and D-bits separately and enhances performance during error bursts.

If an NE does not meet the "8 of 10" objective, it must make theincrement decision based on a majority vote of I-bits, and the decrementdecision based on a majority vote of D-bits.

New Data Flag (NDF).

Bits 1 through 4 (N-bits) of the pointer word carry an NDF, which allowsan arbitrary change of the pointer value due to a change in the payload.

Normal operation is indicated by a "0110" code in the N-bits asillustrated in FIG. 5. The NDF is set by inverting the N-bits to "1001".The new alignment of the STS SPE is indicated by the pointer valueaccompanying the NDF and takes effect at the offset indicated.

The decoding at the receiver is performed by majority voting (i.e. theNDF is considered set if three or more N-bits match to "1001").

Concatenation Indicator

A concatenation indicator contained in the STS-1 payload pointer is usedto show that the STS-1 is part of an STS--Nc.

The first STS-1 within an STS--Nc has a normal pointer word. Allsubsequent STS-1s within the group have their pointer values (bits 7through 16) set to all ones, the NDF set to "1001", and the remainingbits set to zeros. This value of the pointer word does not indicate avalid offset, but is a concatenation indicator. When the pointerprocessors interpret this value, they perform the same operations asperformed on the first STS-1 within the STS--Nc.

Pointer Generation

The STS-1 Pointer is generated according to the rules listed below.

1. During normal operation, the pointer locates the start of the STS SPEwithin the STS Envelope Capacity. The NDF has the normal value "0110".

2. The pointer value can only be changed by operations 4, 5 or 6.

3. If an STS--Nc envelope is being transmitted, a pointer is generatedfor the first STS-1 only. The Concatenation Indicator is generated inplace of the other pointers. All operations indicated by the pointer inthe first STS-1 apply to each STS-1 in the STS--Nc.

4. If a positive stuff is required, the current pointer value is sentwith the I-bits inverted, and the subsequent positive stuff opportunityis filled with dummy information. Subsequent pointers contain theprevious pointer value incremented by one. No subsequent increment ordecrement operation is allowed for three frames following thisoperation.

5. If a negative stuff is required, the current pointer value is sentwith the D-bits inverted, and the subsequent negative stuff opportunityis overwritten with an SPE byte. Subsequent pointers contain theprevious pointer value decremented by one. No subsequent increment ordecrement operation is allowed for three frames following thisoperation.

6. If the alignment of the envelope changes for any reason other thanrules 4 or 5, the new pointer value shall be sent accompanied by the NDFset to "1001". The set NDF only appears in the first frame that containsthe new value. The new envelope begins at the first occurrence of theoffset indicated by the new pointer. No subsequent increment ordecrement operation is allowed for three frames following thisoperation.

Pointer Interpretation

The STS-1 pointer is interpreted according to the rules listed below.

1. During normal operation, the pointer locates the start of the STS SPEwithin the STS Envelope Capacity.

2. Any variation from the current pointer value is ignored unless aconsistent new value is received three times consecutively or it ispreceded by rule 4, 5 or 6. Any consistent new value received threetimes in succession overrides (i.e., takes priority over) rules 4, 5 or6.

3. If the pointer word contains the Concatenation Indicator, then theoperations performed on the STS-1 are identical to those performed onthe first STS-1 within the STS--Nc. Rules 4 and 5 do not apply to thispointer word.

4. If a positive stuff operation is indicated, the current pointer valueshall be incremented by one.

5. If a negative stuff operation is indicated, the current pointer valueshall be decremented by one.

6. If NDF set is indicated, then the coincident pointer value shallreplace the current one at the offset indicated by the new pointervalue.

VT Payload Pointers

The VT Payload Pointer provides a method of allowing flexible anddynamic alignment of the VT SPE within the VT Superframe, independent ofthe actual contents of the envelope.

VT Pointer Value

The V1 and V2 bytes allocated to the VT pointer can be viewed as oneword, as FIG. 9 shows.

The pointer value (bits 7 through 16) is a binary number that indicatesthe offset from V2 to the first byte of the VT SPE. The range of theoffset is different for each of the VT sizes, as FIG. 10 illustrates.The pointer bytes are not counted in the offset calculation.

Frequency Justification

The VT Payload Pointer is used to frequency justify the VT SPE to theSTS SPE rate in exactly the same way that the STS-1 Payload Pointer isused to frequency justify the STS SPE to the frame rate of the TransportOverhead. A positive stuff opportunity immediately follows the V3 byte.V3 serves as the negative stuff opportunity, such that when theopportunity is taken, the V3 byte is overwritten by an SPE byte. This isillustrated in FIG. 10. The indication of whether or not a stuffopportunity has been taken is provided by the I- and D-bits of thepointer in the same VT Superframe. The value contained in V3 when notbeing used for a negative stuff is undefined.

VT Size Indicator

Bits 5 and 6 of the VT Payload Pointer indicate the size (x) of the VTx.Four sizes are defined along with their corresponding VT pointer range,as Table II shows.

                  TABLE II                                                        ______________________________________                                        VT Size Indicator                                                             Size       Designation                                                                             VT Pointer Range                                         ______________________________________                                        00         VT6       0-427                                                    01         VT3       0-211                                                    10         VT2       0-139                                                    11           VT1.5   0-103                                                    ______________________________________                                    

New Data Flag (NDF)

Bits 1 through 4 (N-bits) of the pointer word carry an NDF. It allows anarbitrary change of the value of a pointer, or the size of the VT, dueto a change in the payload. If there is a change in VT size, then,implicitly, there is a simultaneous new data transition in all of theVTs in the VT group.

Normal operation is indicated by "0110" in the N-bits. NDF shall be setto "1001" to indicate a new alignment for the envelope, or a new size.If a new size is indicated, then all VT pointers (1 to 4) in the VTgroup shall simultaneously indicated NDF with the same new size. The newalignment, and possibly size, is indicated by the pointer value and sizeindicator accompanying the NDF and takes effect at the offset indicated.

The decoding at the receiver is performed by majority voting (i.e., theNDF is considered set if three or more N-bits match "1001").

VT Concatenation

Sub-STS-1 services that require N times VTx rates can be transported byconcatenated VTx--NCs whose constituent VTxs are kept together in a wayto ensure phases and sequence integrity.

As with the STS-1 Payload Pointer the VT Payload Pointer uses theConcatenation Indicator (all ones in bits 7 through 16, the NDF set to"1001", and VT sizes to "00") to indicate that the VTxs areconcatenated. This function is only defined for VT6s in the formation ofVT6--Ncs. If a pointer contains the Concatenation Indicator, it is anindication to the pointer processor that this VTx is concatenated to theprevious VTx, and all operations indicated by the previous pointer areto be performed on this VTx as well.

VT Pointer Generation and Interpretation

Generation and interpretation of the VT Payload Pointer for VT SPEfollows the rules provided for the STS-1 Payload Pointer, with thefollowing modifications:

1. The terms "STS" and "frame" are replaced with "VT" and "superframe",respectively.

2. Additional pointer generation rule 7: If the size of the VTs within aVT group is to change, then an NDF, as described in rule 6, shall besent in all VTs of the new size in the group simultaneously.

3. Additional pointer interpretation rule 7: if a set NDF and anarbitrary new size of VT are received simultaneously in all of the VTswithin a VT group, then the coincident pointers and sizes shall replacethe current values immediately.

Performance Monitoring

Performance Monitoring (PM) is required according to the level offunctionality provided in the NE. SONET provides the capability ofgathering PM data based on overhead bits, such as Bit InterleavedParity--N (BIP--N) bits, at the Section, Line and Path layers. For thoseSONET NEs that interface to the existing digital network (e.g., DS1),certain DSn PM parameters may also be required.

PM refers to the in-service monitoring of transmission quality. Thethree key components of PM are:

1. Detection of transmission degradation;

2. Derivation of useful performance parameters from the detecteddegradation; and

3. Communication of these parameters to a surveillance OS.

Performance parameters to be monitored are described herein. Thresholdson these parameters are used to detect transmission degradation.Threshold Crossing Alerts (TCAs) are used to notify a surveillance OS ofa degradation. The OS may also query the NE for performance informationstored in the NE.

BELLCORE TECHNICAL REFERENCE NO. TR-TSY-000820 specifies PM strategiesand a general set of PM parameters. SONET requires modification of someexisting parameters and the definition of some new parameters. Thefollowing sections describe the PM parameters for each SONET layer, andidentify the storage requirements for SONET NEs. At certain SONET layers(e.g., STS and VT paths), both near end and far end performance can bemonitored. Where far end performance can also be monitored, appropriateperformance parameters, storage and thresholding requirements areprovided.

Section layer performance parameters available in SONET are:

1. Severely Errored Framing Second (SEFS)--This parameter countsoccurrences of an OOF detected by an on-line framer, or occurrences ofan off-line framer changing frame alignment to a new frame position Inmeasuring SEFSs, a 1-second integration period is allowed so that one ormore OOF/COFAs (Change-of-Frame-Alignments) occurring within a 1-secondinterval count as one SEFS event.

2. Section Coding Violations (CVs)--Section CVs are BIP errors that aredetected at the Section layer of the incoming SONET signal. The SectionCV counter is incremented for each BIP error detected. That is, eachBIP-8 can detect up to 8 errors per STS--N frame, with each errorincrementing the CV counter. CVs for the Section layer are collectedusing the BIP-8 in the B1 byte located in the Section overhead of STS-1number 1.

3. Section Errored Seconds (ESs)--A Section ES is a second during whichat least one Section CV or OOF/COFA event occurred, or a second duringwhich the NE was (at any point during the second) in the LOS state.

Section Severely Errored Seconds (SESs)--A Section SES is a second withK or more Section CVs, or a second during which at least one OOF/COFAevent occurred, or a second during which the NE was (at any point duringthe second) in the LOS state. The number of Section CVs within a secondthat constitutes a Section SES is settable. Table III defines thedefault values for K for the different OC--N rates. At an OC-1 rate,this definition of a Section SES corresponds to a 1.5×10⁻⁷ BER. At anOC-3 rate an above, this definition of a Section SES corresponds to a10⁻⁷ BER.

                  TABLE III                                                       ______________________________________                                        Section SESs                                                                          Rate  CVs                                                             ______________________________________                                                OC-1   9                                                                      OC-3   16                                                                     OC-12  63                                                                     OC-24 125                                                                     OC-48 249                                                             ______________________________________                                    

There are no far end performance parameters defined for the Sectionlayer.

The Line layer performance parameter available for SONET are:

1. Line CVs--Line CVs are the sum of the BIP errors detected at the Linelayer of the incoming SONET signal. The Line CV counter is incrementedfor each BIP error detected. That is, each line BIP-8 can detect up toeight errors per STS-1 frame, with each error incrementing the CVcounter. CVs for the Line layer are collected using the BIP-8 codes inB2 bytes located in the Line overhead of each STS-1 (because all CVs onan STS--N line are counted together, this is equivalent to counting eacherror in the BIP-8N contained in the B2 bytes of the STS--N lineoverhead). Thus, on an STS--N signal, up to 8×N CVs may occur in aframe.

2. Line ESs--A Line ES is a second during which at least one Line CVoccurred, or a second during which the line was (at any point during thesecond) in the Line AIS state.

3. Line SESs--A Line SES is a second with K or more line CVs or a secondduring which the line was (at any point during the second) in the LineAIS state. The number of Line CVs within a second that constitutes anSES is settable. Table IV defines the default values for K for thedifferent SONET line rates. These definitions of Line SES correspond toa 2×10⁻⁷ BER.

                  TABLE IV                                                        ______________________________________                                        Line SESs                                                                             Rate  CVs                                                             ______________________________________                                                OC-1   12                                                                     OC-3   32                                                                     OC-12 124                                                                     OC-24 248                                                                     OC-48 494                                                             ______________________________________                                    

4. STS Pointer Justifications (PJs)--The STS PJ parameter is a count ofthe differences between incoming pointer justifications detected by anNE on an STS SPE that is not terminated, and the outgoing pointerjustifications performed by the NE on the same STS SPE. A certain numberof pointer justifications are expected during normal operations, butexcessive pointer justification may indicate a synchronization problem.

5. Protection Switching Counts (PSCs)--This parameter applies to eachworking and protection line. This parameter counts the number of timesthat service on a monitored line has been switched to the protectionline and from the protection line.

6. Protection Switching Duration (PSD)--This parameter applies to eachworking and protection line. This parameter counts the number of secondsthat service was switched from the line being monitored to theprotection line. It does not apply to NEs using non-revertive protectionswitching.

The near end (or incoming) STS Path layer performance parametersavailable in SONET are:

1. STS Path CVs--STS Path CVs are BIP errors that are detected at theSTS Path layer of the incoming SONET signal. As with Line and SectionCVs, the STS Path CV counter is incremented for each BIP error detected.CVs for the STS Path layer are collected using the BIP-8 in the B3 bytelocated in the STS Path Overhead.

2. STS Path ESs--An STS Path ES is a second during which at least oneSTS Path CV occurred, or a second during which the NE was (at any pointduring the second) in the STS path AIS state or STS Path LOP state.

3. STS Path SESs--An STS Path SES is a second with K or more STS PathCVs, or a second during which the NE was (at any point during thesecond) in the STS Path AIS state or the STS Path LOP state. The numberof STS Path CVs within a second that constitute an SES is settable.Table V specifies the default values for K for the STS-1 and STS-3c SPE.The specified values correspond to a 1.5×10⁻⁷ BER for the STS-1 SPE anda 10⁻⁷ BER for the STS-3c SPE.

                  TABLE V                                                         ______________________________________                                        STS Path SESs                                                                         Rate  CVs                                                             ______________________________________                                                STS-1  9                                                                      STS-3c                                                                              16                                                              ______________________________________                                    

4. STS Path Unavailable Seconds (UASs)--This parameter is a measure ofduration in seconds for which the STS Path is considered unavailable.FIG. 11 describes the process for determining the beginning ofunavailable time for an incoming STS Path using STS path SESs anddeclared failure indications. FIG. 12 describes the process fordetermining the end of unavailable time for an incoming STS Path. Thefailure conditions referred to in these figures are STS Path AIS and STSPath LOP.

5. VT PJs--The VT PJ parameter is a count of the differences betweenincoming pointer justifications detected by an NE on a VT SPE that isnot terminated, and the outgoing pointer justifications performed by theNE on the same VT SPE. A certain number of PJs are expected duringnormal operations, but excessive pointer justification may indicate asynchronization problem.

Far end performance parameters are also defined for the STS Path layer.Far end STS Path layer performance is conveyed back to the near end STSPTE via the Path Status (G1) byte. Bits 1 through 4 provide an STS PathFar End Block Error (FEBE) indication and convey the number of BIPerrors detected at the far end using the STS Path BIP-8 code in the B3byte. Bit 5 of G1 is an STS Path Yellow indicator. Bits 6, 7 and 8 ofthe G1 byte are currently unassigned.

Implementations of an STS Path FERF function are currently under studyin Committee T1. When finalized, it is expected that modifications willbe made to include requirements related to such a function. One area forwhich an STS Path FERF function would have significant impact is thedefinition and operation of far end STS Path performance parameters. Theavailability of an STS Path FERF function would allow near end and farend counts of PM parameters to be more closely aligned. Until progressis made in standards for the STS Path FERF function, the followingdefinitions apply for far end STS Path performance parameters:

1. Far End STS Path CVs--Far End STS Path CVs are BIP errors detected atthe STS Path layer of the incoming signal at the far end. These CVs arecollected using the STS Path FEBE indication in the incoming STS pathStatus (G1) byte. The Far End STS Path CV counter is incremented foreach error indicated in the STS Path FEBE indication.

2. Far End STS Path ESs--A Far End STS Path ES is a second during whichat least one Far End STS Path CV is counted.

3. Far End STS Path SESs--A Far End STS Path SES is a second duringwhich K or more Far End STS Path CVs are counted. The number of Far EndSTS Path CVs within a second that constitute a Far End STS Path SES issettable. The value of K is identical to the corresponding value fornear end STS Path SESs. Table V specifies the default values for K.

4. Far End STS path UASs--This parameter is a measure (at the near end)of the duration in seconds for which the STS Path is consideredunavailable at the far end. FIG. 13 describes the process fordetermining the beginning of unavailable time for a far end STS Pathusing Far End STS Path SESs and STS Path Yellow signal. FIG. 14describes the process for determining the end of unavailable time for afar end STS Path.

VT Path layer parameters in SONET exist only for floating VTs becausethey are derived from the V5 byte. The near end (or incoming) VT Pathlayer performance parameters available in SONET are:

1. VT Path CVs--VT Path CVs are BIP errors that are detected at the VTPath layer of the incoming SONET signal. CV counters are incremented foreach BIP error detected. CVs for the VT Path layer are collected usingthe BIP-2 in the V5 overhead byte of the floating VT.

2. VT Path ESs--A VT Path ES is a second during which at least one VTPath CV occurred, or a second during which the NE was (at any pointduring the second) in the VT Path AIS state or VT Path LOP state.

3. VT Path SESs--A VT Path SES is a second with K or more VT Path CVs,or a second during which the NE was (at any point during the second) inthe VT Path AIS state or VT Path LOP state. The number of VT Path CVswithin a second that constitutes a VT Path SES is settable. Table VIdefines the default values for K for the different size VTs. Thespecified default values correspond to a 2×10⁻⁶ BER.

                  TABLE VI                                                        ______________________________________                                        VT Path SESs                                                                          Rate   CVs                                                            ______________________________________                                                  VT1.5                                                                              4                                                                      VT2    6                                                                      VT3    8                                                                      VT6    14                                                             ______________________________________                                    

4. VT Path UASs--This parameter is a measure of duration in seconds forwhich the VT Path is considered unavailable. FIG. 11 describes theprocess for determining the beginning of unavailable time for anincoming VT Path using VT Path SESs and declared failure indications.FIG. 12 describes the process for determining the end of unavailabletime for an incoming VT Path. The failure conditions referred to inthese figures are VT Path AIS and VT Path LOP.

Far end performance parameters are also defined for the VT Path layerfor floating VTs. Far end VT Path layer performance is conveyed back tothe near end VT PTE via the V5 VT Path overhead byte. Bit 3 provides aVT Path layer FEBE indication and bit 8 provides a VT Path Yellowindicator. Bit 4 of the V5 byte is currently unassigned.

Implementations of a VT Path FERF function are currently under study inCommittee T1. When finalized, it is expected that modifications will bemade to include requirements related to such a function. One area forwhich a VT Path FERF function would have significant impact is thedefinition and operation of far end VT path performance parameters. Theavailability of a VT Path FERF function would allow near end and far endcounts of PM parameters to be more consistent. Until progress is made instandards for the VT Path FERF function, the following definitions applyfor far end VT Path performance parameters:

1. Far End VT Path CVs--Far End VT Path CVs are BIP errors detected atthe VT Path layer of the incoming signal at the far end. These CVs arecollected using the VT Path FEBE indication in the incoming V5 byte. TheFar End VT Path CV counter is incremented for each error indicated inthe VT Path FEBE indication.

2. Far End VT Path ESs--A Far End VT Path ES is a second during which atleast one Far End VT Path CV is counted.

3. Far End VT Path SESs--A Far End VT Path SES is a second during whichK or more Far End VT Path CVs are counted. The number of Far End VT PathCVs within a second that constitute a Far End VT Path SES is settable.The value of K is identical to the corresponding value for near end VTPath SESs. Table VI specifies the default values for K.

4. Far End VT Path UASs--This parameter is a measure (at the near end)of the duration in seconds for which the VT Path is consideredunavailable at the far end. FIG. 13 describes the process fordetermining the beginning of unavailable time for a far end VT Pathusing Far End VT Path SESs and VT Path Yellow signal. FIG. 14 describesthe process for determining the end of unavailable time for a far end VTPath.

The accumulation and storage of most performance parameters areinhibited during periods of unavailability for monitored entities whereunavailable seconds is a defined parameter. Inhibiting a performanceparameter during periods of unavailability is accomplished by notincrementing the current 15-minute and current-day registers by thevalue of the parameter for the second during which the monitored entityis considered unavailable (i.e. overwrite with zero foraccumulation/storage purposes). Therefore, the only counts that shouldappear in the 15-minute and daily storage registers for a givenparameter that is inhibited during periods of unavailability are countsfor that parameter during times of availability of the monitored entity.UAS for a monitored entity (where it is defined) is never inhibited andit keeps track of unavailable time.

The accumulation and storage of CVs for a monitored entity is alsoinhibited in the same manner during certain periods of availability;that is for seconds during which the NE was (at any point during thesecond) in certain failure states for the monitored entity.

The rules for inhibiting performance parameters (including the far endparameters when activated) are summarized in the following requirements:

1. At the Section, Line, STS Path and VT Path layers, counts of near endCVs at the layer shall be inhibited during all seconds during which themonitored entity is (at any point during the second) in a LOS or LOFstate. At the line, STS Path and VT Path layers, these counts shall alsobe inhibited during all seconds during which the monitored entity is inan AIS state. At the STS Path and VT Path layers, these counts shallalso be inhibited during all seconds during which the monitored entityis in the LOP state. Far end Path CVs, as currently defined for the STSPath and VT Path layers, are not inhibited in the above manner sincethere is currently no mechanism available for the near end to know whenthe far end is experiencing Path AIS or Path LOP. Far end Path CVs areinhibited during unavailability of the path at the far end (seerequirements below).

2. At the STS Path and VT Path layers, all near end performanceparameters except for UASs are inhibited during periods ofunavailability of the incoming path signal (see FIGS. 11 and 12 fordetermining beginning and end of unavailable time for incoming signalsat these layers).

3. When the accumulation of far end STS Path or VT Path performanceparameters at the near end is activated, all far end parameters exceptfor far end UASs are inhibited during periods of unavailability of thepath signal at the far end (see FIGS. 13 and 14 for determiningbeginning and end of unavailable time for far end paths at theselayers).

4. When the accumulation of far end STS Path performance parameters atthe near end is activated, to ensure valid far end monitoring, far endparameters shall be derived and accumulated only during 1-secondintervals during which the near end NE is not (at any point during thesecond) in the STS Path LOP state or STS Path AIS state for the incomingsignal. For those 1-second intervals during which the NE is in the STSPath LOP state or STS Path AIS state for the incoming signal, all farend parameters are overwritten to zero and such counts are flagged asincomplete when queries for that data are processed since such zerocounts may not truly reflect the performance experienced at the far end.

5. When the accumulation of far end VT Path performance parameters atthe near end is activated, to ensure valid far end monitoring, far endparameters are derived and accumulated only during 1-second intervalsduring which the near end NE is not (at any point during the second) inthe VT Path LOP state or VT Path AIS state for the incoming signal. Forthose 1-second intervals during which the NE is in the VT Path LOP stateor VT Path AIS state for the incoming signal, all far end parameters areoverwritten to zero and such counts are flagged as incomplete whenqueries for that data are processed since such zero counts may not trulyreflect the performance experienced at the far end.

Table VII depicts the requirements for PM during troubles according tothe requirements above.

                                      TABLE VII                                   __________________________________________________________________________    SONET PM Accumulation During Troubles                                                   Trouble                                                                                   STS                                                                              STS    VT VT                                                         Line                                                                             STS                                                                              Path                                                                             Path                                                                              VT Path                                                                             Path                                       Parameter LOS                                                                              LOF                                                                              AIS                                                                              LOP                                                                              AIS                                                                              Yellow                                                                            LOP                                                                              AIS                                                                              Yellow                                     __________________________________________________________________________    Section SEFSs                                                                           y  y  y  y  y  y   y  y  y                                          Section CVs                                                                             N  N  y  y  y  y   y  y  y                                          Section ESs                                                                             y  y  y  y  y  y   y  y  y                                          Section SESs                                                                            y  y  y  y  y  y   y  y  y                                          Line CVs  N  N  N  y  y  y   y  y  y                                          Line ESs  y  y  y  y  y  y   y  y  y                                          Line SESs y  y  y  y  y  y   y  y  y                                          STS PJs   n  n  n  n  n  y   y  y  y                                          PSCs      y  y  y  y  y  y   y  y  y                                          PSD       y  y  y  y  y  y   y  y  y                                          STS Path CVs                                                                            N  N  N  N  N  y   y  y  y                                          STS Path ESs                                                                            n  n  n  n  n  y   y  y  y                                          STS Path SESs                                                                           n  n  n  n  n  y   y  y  y                                          VT PJs    n  n  n  n  n  y   n  n  y                                          STS Path UASs                                                                           y  y  y  y  y  y   y  y  y                                          Far End STS CVs                                                                         O  O  O  O  O  n   y  y  y                                          Far End STS ESs                                                                         O  O  O  O  O  n   y  y  y                                          Far End STA SESs                                                                        O  O  O  O  O  n   y  y  y                                          Far End STS UASs                                                                        O  O  O  O  O  y   y  y  y                                          VT Path CVs                                                                             N  N  N  N  N  y   N  N  y                                          VT Path ESs                                                                             n  n  n  n  n  y   n  n  y                                          VT Path SESs                                                                            n  n  n  n  n  y   n  n  y                                          VT Path UASs                                                                            y  y  y  y  y  y   y  y  y                                          Far End VT CVs                                                                          O  O  O  O  O  n   O  O  n                                          Far End VT ESs                                                                          O  O  O  O  O  n   O  O  n                                          Far End VT SESs                                                                         O  O  O  O  O  n   O  O  n                                          Far End VT UASs                                                                         O  O  O  O  O  y   O  O  y                                          __________________________________________________________________________     y  indicates the parameter shall continue to be counted/accumulated for       seconds during which the trouble is present.                                  N  indicates the parameter shall be inhibited for all seconds during whic     the trouble is (at any point during the second) detected as well as durin     periods of unavailability.                                                    n  indicates that the parameter shall be inhibited during periods of          unavailability.                                                               O  indicates the parameter shall be counted as zero and marked as             incomplete for each second during which the trouble is (at any point          during the second) detected on the incoming signal.                      

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and system fortesting SONET network elements so that it can be quickly and easilydetermined that the network elements conform to various acceptedcriteria and/or standards.

In carrying out the above object and other objects of the presentinvention, a method is provided for testing a SONET network elementhaving performance monitoring parameters defined for at least onelogical layer and capable of performing at least one performancemonitoring function. The network element has a normal operating stateand a trouble state. The method includes the steps of generating a basictest signal, modifying the basic test signal to obtain a modified testsignal, and detecting the modified test signal at the network element tocause the network element to change from the normal operating state tothe trouble state. The network element initiates a set of actions inresponse to the modified test signal. The method also includes the stepof monitoring the modified test signal to determine whether the networkelement is to change back from the trouble state to the normal operatingstate. The modified test signal includes a first error portion which isfollowed by a first error-free portion having a first predetermined timeperiod which is followed by a trouble portion having a periodsufficiently long to be detected and cause the network element to changefrom the normal operating state to the trouble state. The troubleportion is followed by a second error-free portion having a secondpredetermined time period which is followed by the second error portion.The modified test signal tests at least one logical layer. The set ofactions includes inhibiting accumulation of the parameters during a timeperiod in which the network element is in the trouble state andaccumulating parameters associated with the first and second errorportions of the modified test signal.

Further, in carrying out the above object and other objects of thepresent invention, a method is provided for testing a SONET networkelement including the step of generating a test signal including apayload pointer initially having a first pointer value and subsequentlyhaving a second pointer value different from the first pointer value.The payload pointer indicates the location of the beginning of anincoming synchronous payload envelope (SPE) of the test signal in orderto extract a payload signal therefrom. The payload signal includesinformation bytes having a predetermined pattern. The method alsoincludes the steps of detecting the test signal at the network elementso that the network element extracts the payload signal from the SPE andto detect the information bytes having the predetermined pattern, anddetermining a delay between a first occurrence of the second pointervalue and a time when the extracted payload signal includes only theinformation bytes having the predetermined pattern. The delay indicateshow long the network element delays in using the second pointer value.

Systems are also provided for carrying out the above method steps.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating various NEs andinterface types in SONET operations communications;

FIG. 2 is a schematic view illustrating the synchronouspayload/envelopes (SPE) of a STS-1 frame;

FIG. 3 is a schematic block diagram illustrating SONET sections, lineand path definitions;

FIG. 4 is a schematic block diagram illustrating various opticalinterface layers;

FIG. 5 is a schematic diagram illustrating STS-1 Payload Pointer (H1,H2, H3) Coding;

FIG. 6 is a schematic diagram illustrating STS-1 Pointer OffsetNumbering;

FIG. 7 is a schematic diagram illustrating positive STS-1 PointerJustification Operation;

FIG. 8 is a schematic diagram illustrating negative STS-1 PointerJustification Operation;

FIG. 9 is a schematic diagram illustrating VT payload pointer coding;

FIG. 10 is a schematic diagram illustrating VT pointer offsets;

FIG. 11 is a block diagram flow chart illustrating a process fordetermining the beginning of unavailable time for an incoming STS or VTpath;

FIG. 12 is a block diagram flow chart illustrating a process fordetermining the end of unavailable time for an incoming STS or VT path;

FIG. 13 is a block diagram flow chart illustrating a process fordetermining the beginning of unavailable time for a far end STS or VTpath;

FIG. 14 is a block diagram flow chart illustrating a process fordetermining the end of unavailable time for a far end STS or VT path;

FIG. 15 is a block diagram flow chart illustrating one method of thepresent invention;

FIG. 16 is a block diagram flow chart illustrating a second method ofthe present invention; and

FIG. 17 is a diagram showing an example VT NDF test signal andcorresponding results.

BEST MODE FOR CARRYING OUT THE INVENTION

In general, the methods and systems of the present invention are capableof testing SONET network elements (NEs) against selected criteriacontained in TR--NWT-000253, Synchronous Optical Network (SONET)Transport Systems: Common Generic Criteria (TR 253). The SONET test setfeatures include a "basic" signal sent by the test set and the easewhich the basic signal can be modified to perform the selected tests orto test different types of NEs.

The Basic Signal

One of the most important considerations in testing SONET NEs is the"basic" signal sent by the SONET test set. In general, the tests areperformed by changing back and forth between a basic signal and a copyof that signal that has been altered so that it contains someabnormality (e.g., an errored framing pattern, or an all-ones STSpointer word) that the NE is required to detect as an incoming failureor maintenance signal. Any DSn payloads contained in the basic signalare identifiable and detectable on typical DSn test sets.

The basic signal should comply with the criteria in TR 253, Section 3.All of the Section, Line, STS Path, and VT Path (if applicable) overheadbytes contain the required values, all of the "unused" bits and bytesare set to zeros, and the payload mappings are as shown in TR 253. Whilethere is no single basic signal that can be used to test all NEs,factors such as the particular payload mappings and protection switchingmodes supported by an NE determine the contents of the basic signalneeded to test that NE.

STS and VT Pointers--Pointer Generation and Interpretation

Several of the pointer interpretation criteria contain specificdetection limits. The limits are given in terms of the number of bits inthe pointer word that are inverted from their previous or normal values.If the specified limit (or more) of the appropriate bits are inverted,then the NE is supposed to detect them as an incoming pointer change andtake certain immediate actions, but if fewer bits than the limit areinverted, then the NE should ignore them (except for the purpose ofdetermining if it should enter the LOP state).

SONET Performance Monitoring

In general, similar tests need to be performed for each layer of SONETPM supported by the NE, so the capability to perform error insertion ateach layer is very important, as is the capability to insert errors atprecise rates.

A number of the PM criteria in TR 253 contain detection limits. Some ofthese limits are given in terms of the number of coding violations thatare detected in a second, while others are given in terms of the time aparticular type of signal persists. In general, several tests should beperformed for each of these criteria. In one test, a signal with exactlythe limit for a particular item should be sent to the NE and the NEshould be monitored to determine if it takes the appropriate actions(e.g., inhibits the accumulation of certain parameters), while inanother test a signal should be sent with just less than the limit tomake sure the NE does not take those actions.

General Considerations Test Set Error Insertion Capabilities

Although the insertion of an error into a SONET signal is a very simpleconcept (i.e., on the signal transmitted by the test set a bit that issupposed to be a "1" gets changed to a "0", or a "0" to a "1"), thereare many possible ways that the insertion can be performed. Forinstance, the errors could be inserted on particular bits within theSONET frame as specified by the user, or they could be inserted"randomly" throughout the signal. Also, the layers at which the insertederrors will cause Coding Violations (CVs) to be counted by an NE dependson the functionality of the particular test set. For instance, if thetest set inserts an error in the VT SPE before it calculates any of theBIP codes, then that error would appear on the DS1 dropped from thatparticular VT, but it would not cause any CVs to be accumulated by theNE. Conversely, if the test set inserts the error after all of the BIPshave been calculated, then it will cause CVs at all of the SONET layersin addition to the DS1 payload error. Several types of error insertionin between these two extremes are also possible, and each method has itsown advantages and disadvantages. In general, the PM test procedures canbe adapted to the type (or types) of error insertion supported by aparticular test set.

In many PM tests, the number of CVs accumulated by the NE should becompared to the number of errors inserted by the test set. If the testset provides precise control of the length of the error insertionperiod, then the number of errors may be calculated from the insertiontime and the error rate. However, if the error insertion period cannotbe precisely controlled, then some other method of determining thenumber of errors is needed. One possibility is that the test set couldbe designed to display the number of transmitted errors. If that featureis not provided but the test set normally counts the CVs on the signalthat it receives, then the number of errors sent could be determined bysplitting the optical signal transmitted by the test set and looping onepart of that signal back to the test set's receiver, while sending theother part to the NE under test.

Error Insertion Timing Issues

In many PM tests, it is not possible to give specific values for thenumber of counts that should be accumulated by the NE during the test.The reasons for this is that it is generally not possible to control therelative timing of the error insertion and the NE's PM clock. Forexample, although it may be possible for the test set to insert oneerror every 250 frames for exactly 1 second (i.e., 32 errors in 8000frames), there is no way to make sure that the NE will detect all 32 ofthe resulting Line CVs as occurring in a single PM second. In thisexample, it is much more likely that the NE will detect some of theerrors at the end of one second, and the rest of them at the beginningof the next second. Thus, the NE would accumulate 2 ESs even though thesignal was only errored for one second.

Another factor that affects the expected results in various PM tests isthe burstiness of the errors. An example of this appears in the expectedES and Severely Errored Second (SES) counts in the Line SES accumulationtest described in Table VIII. In that test, 32 errors per second (thatthe NE will detect as Line CVs) need to be inserted. Since the maximumnumber of Line CVs that can be detected in a single OC-3 frame is 24,the errors have to be inserted in 2 or more SONET frames. If the errorsare inserted in bursts (e.g. 16 errors in each of two consecutiveframes, followed by 7998 error-free frames), then it is expected that inmost cases the NE will detect exactly 32 errors during each second ofthe test, in which case it should count exactly 6 ESs and 6 SESs (for a6 second test). However, if the test set spreads the errors out over theentire second, it is likely that the start of the test will occursometime in the middle of a second and end 6 seconds later in the middleof another second, so those two seconds will each contain less than 32errors. Therefore, the expected result for this case is that the NEshould accumulate 7 ESs and only 5 SESs. Also, in the case where theerrors are spread out over the entire second, a frequency offset betweenthe timing references for the NE and the test set could cause the NE tooccasionally detect only 31 errors in a second. For that reason, to testthe upper detection limit it is recommended that an average error rateslightly greater than the limit (e.g. 32.1 CVs/second) be used instead.Similarly, to test the NE's response to errors just under the limit, itis recommended that an average error rate slightly less that the limit(e.g. 30.9 CVs/second) be used. (Note that for an error rate of 30.9CVs/second, 9 out of every 10 seconds would be expected to contain 31errors, and the remaining second would contain 30 errors).

                                      TABLE VIII                                  __________________________________________________________________________    SONET Performance Monitoring, General                                         Example Criteria.sup.1                                                                           Test Method                                                __________________________________________________________________________    (R) All possible numbers of BIP                                                                  Send a signal containing a single frame with 1 B2                             error. The NE must                                         errors (e.g. 0 to 24 errors in a                                                                 accumulate 1 Line CV, and 1 ES.                            single frame for the OC-3 Line                                                                   Send a signal containing a single frame with 24 B2                            errors. The NE must                                        Layer), are detected and accumulated                                                             accumulate 24 Line CVs, and 1 ES.                          as CVs.                                                                       (R) For an OC-3 line, a second                                                                   Send a signal containing periodic B2 errors at a rate                         of 32 per second for                                       containing 32 or more Line CVs is a                                                              6 seconds.sup.2. The NE must accumulate the correct                           total number of Line CVs,                                  Line SES (and a Line ES).                                                                        7 or 6 ESs, and 5 or 6 SESs..sup.3                                            Send a signal containing periodic B2 errors at a rate                         of 31 per second for 6                                                        seconds. The NE must accumulate the correct total                             number of Line CVs, 7 or 6                                                    ESs, and 0 SESs..sup.3                                     (R) An NE with an off-line framer                                                                Send a signal with x- 1 consecutive frames containing                         single bit errors in                                       accumulates a Section SEFS for any                                                               the framing pattern..sup.4 The NE must accumulate the                         correct number of Section                                  second in which it detects a COFA (or                                                            CVs and ESs, and 0 SEFSs and SESs..sup.5                   is in the LOS or LOF state).                                                                     Send a signal containing an arbitrary change in the                           frame alignment.                                           (R) An NE with an on-line framer                                                                 The NE must accumulate 1 Section SEFS, 1 ES, and 1                            SES. It should also                                        accumulates a Section SEFS for any                                                               accumulate a burst of CVs. (CVs should be detected in                         approximately x+2                                          second in which it is in the OOF (or                                                             frames, with a maximum of 8 CVs per frame).                LOS or LOF) state. Send a signal with x consecutive frames containing                            single bit errors in the                                                      framing pattern. An NE with an on-line framer must                            accumulate 1 Section                                                          SEFS, ES, and SES, and also a burst of CVs. An NE with                        an off-line framer                                                            must accumulate the correct number of CV and ESs, and                         0 SEFSs and SESs..sup.5                                                       Send a signal containing a short (e.g., 4 ms) LOS or                          LOF. The NE must                                                              accumulate 0 Section CVs, and 1 SEFS, ES and SES.                             Send a signal containing a 20 second LOS or LOF. The                          NE must accumulate                                                            0 CVs, and 20 or 21 SEFS, ESs and SESs.                    __________________________________________________________________________     .sup.1 Criteria statements from TR 253 are paraphrased below.                 .sup.2 The 6 second test duration given in this example is arbitrary. If      the NE being tested does not accumulate Line UASs, then any convenient        test duration could be used. However, if the NE does accumulate Line UASs     then the test duration should be less than 10 seconds, and the Line UAS       parameter should be tested separately.                                        .sup.3 See Error Insertion Timing Issues.                                     .sup.4 The meaning of "x" depends on whether the NE uses the proposed ANS     definition of a SEFS, or one of the TR 253 definitions. If the ANSI           definition is used, the x is part of the SEFS definition and is equal to      4. If the TR 253 definition is used, then x is the number of consecutive      frames that must contain errored framing patterns for a particular NE to      detect an OOF. In the TR 253 case, x is NE dependent and can equal either     4 or 5.                                                                       .sup.5 The number of CVs, ESs, and SESs accumulated with depend on the        functionality of the test set. If the test set inserts errors into the        framing pattern before the B1 BIP8 code is calculated, then the NE should     detect 0 CVs, ESs and SESs. However, if the test set inserts the errors       after B1 is calculated, then the NE should detect x-1 or x CVs, 1 ES and      SESs.                                                                    

Failure Insertion Timing Issues

Similar to the error insertion timing discussed above in the tests of anNE's PM accumulation during incoming "troubles" (i.e., failures orAISs), there is generally no way to control the timing of the start ofthe trouble with respect to the NE's PM clock. In most tests, thetrouble will be inserted sometime in the middle of a second so that anyresulting CVs will appear to the NE to have occurred in the same secondas the entry into the trouble state (in which case the accumulation ofCVs at certain SONET levels are required to be inhibited). However,there is the possibility that the trouble will begin (and cause CVs)just before the end of one second, but the NE will not enter the troublestate until after the start of the next second. If this occurs, then itwill appear that the NE failed to inhibit the accumulation of CVs duringseconds in which it was in a trouble state, when in fact the CVs weredetected in the previous second.

Another timing issue is the length of time that the failure must beinserted, which is affected by the architecture of the NE. As anexample, one possible architecture for an NE that terminates an OC--Nsignal and provides DS1 interfaces on its low-speed side consists of ahigh speed circuit pack, several medium speed packs, and a number of lowspeed packs. The high speed pack could terminate the OC--N signal andperform STS pointer processing before sending STS-1 signals to themedium speed packs. The medium speed packs could then terminate theSTS-1s and send the VT1.5 signals on to the low speed packs. Finally,the low speed packs could terminate the VTs and output the DS1 traffic.In this architecture, the low speed pack could be responsible fordetecting VT LOP and VT AIS, and for performing the VT Path PM function.Since this pack would not be able to distinguish between a VT AISoriginated by a separate NE and a VT AIS that was inserted by the highor medium speed pack in response to some higher level failure or AIS, itmay not have any way of "knowing" which failure or AIS state the NE (asa whole) is in. Only the knowledge that it (the low speed pack) is inthe VT LOP or AIS state may be available at the device where the VT PathPM is accumulated.

In most cases, the possibility that the device performing the VT Path PMmight not know the state of the NE as a whole should not make adifference in the PM accumulation. For the architecture described above,if the NE is in the LOS state on the active OC--N line, then the lowspeed pack should receive VT AIS (from the medium speed pack) and thecriteria for VT Path PM are the same whether the NE is in the LOS stateor the VT AIS state during a particular second. In either case, TableVII indicates that VT Path CVs must not be accumulated for thoseseconds. The difference could occur for a very short LOS. With thisparticular architecture, the LOS would need to last for more than 12frames for the low speed card to detect the VT AIS state. Thus, if theLOS lasts for less than 12 frames, then the accumulation of VT Path CVsmight not be inhibited. Also, the 12 frames delay between the detectionof LOS at the high speed pack and the detection of VT AIS at the lowspeed pack would increase the chance that the failure would start in onesecond but the entry into the state that causes the CV counts to beinhibited would not occur until the next second.

Protection Switching Considerations For Path PM

When an NE performs a protection switch it is expected that there willbe some effect on the Path PM parameters, and that effect should dependon the cause of the switch. If a Line failure or AIS is the cause of theswitch, then the NE should inhibit the accumulation of Path CVs duringthe second in which the switch occurs, but it should count 1 ES and 1SES on each affected Path (assuming the start of the failure and theswitch occur in the same second). However, if the switch has some othercause (e.g., an SD condition on the active OC--N line), then the NEwould be expected to detect a small burst of Path CVs during the switch,and those CVs should be accumulated along with any resulting ESs andSESs.

As noted previously, when Table VII indicates that the accumulation ofPath CVs must be inhibited for seconds in which the NE is in a Linefailure or AIS state, it is really only referring to Line failures orAISs that are detected on an active OC--N line. Similarly, when thetable indicates that the accumulation of STS and VT path ESs and SESsmust be inhibited during unavailable time caused by a declared Linefailure or AIS condition, it is really only referring to failures orAISs that occur on an active line and where a protection switch does notrestore the Path. If the failure occurs on a standby line, or aprotection switch restores the Path, then the Path is not unavailable.Therefore, any tests related to the accumulation of Path PM parametersduring a declared Line failure or AIS would need to be performed withthe NE configured or provisioned so that a switch cannot occur.

The NE may also have to be configured to prevent a protection switch ina number of other Path PM tests, if errors that are inserted by the testset to cause STS or VT CVs also cause Line CVs. If switches are notprevented for some of these tests, then an undesired switch caused by anSD condition could occur.

Applicability of Tests to Various SONET Layers

In SONET, three common PM parameters (i.e., CVs, ESs, and SESs) aredefined at the Section, Line, STS path and VT Path layers, along withvarious parameters that are only applicable to some of those layers. InTR 253, the additional parameters that are defined are SEFSs at theSection layer, Pointer Justifications (PJs) at the Line and STS Pathlayers, and UASs at the STS and VT Path layers.

In general, if a parameter is defined for more than one layer, then theprocedures for testing the accumulation of that parameter are basicallythe same at each layer. Therefore, when example tests are given, theyoften refer to a specific layer, but the procedures could easily bemodified to test the same parameter at a different layer. Also, most ofthe procedures described, test the NE's accumulation of more than oneparameter. Thus, even though there is not a specific section describingtest procedures for the accumulation of ESs, that capability is testedas part of a number of other tests.

PM During Seconds With Failures And AISs

Table IX lists several example tests that could be used to test an NEagainst the various criteria related to PM during seconds in which theNE is in a failure or AIS (trouble) state, but which are not consideredto be unavailable time. These examples refer specifically to theaccumulation of STS Path parameters, but similar tests could beperformed for the other layers. For the VT Path layer the primarydifference would be that the NE's response to VT LOP and VT AIS shouldalso be tested. At the Line and Section layers, the PM accumulationshould be independent of any protection switches that may occur, so thethird test listed would not be necessary. Also, only Line failuresshould affect the accumulation of Section CVs, and only Line failuresand AIS should affect the Line CVs.

Referring now to FIG. 15, in the first two of the example tests, asingle error is sent one second before the start of the trouble, and twomore errors are sent one second after the end of the trouble. Theseerrors are intended to test if the NE only inhibits the accumulationduring the correct seconds. They should be sent a full second before andafter the trouble so that they cannot be detected as occurring in thesame second as the trouble and the NE will be required to accumulatethem.

                                      TABLE IX                                    __________________________________________________________________________    SONET Performance Monitoring, Inhibition Of CV Accumulation                   Example Criteria                                                                              Test Method                                                   __________________________________________________________________________    (R) The accumulation of STS                                                                   With the standby OC-N line failed or locked                   Path CVs is inhibited during                                                                  out and the alarm delay set for 2.5                           all seconds in which the NE is                                                                seconds, send a signal containing the                         in the LOS, LOF, Line AIS, STS                                                                following:                                                    LOP or STS Path AIS state (at                                                                  1 B3 error                                                   any time during the second).                                                                   1 second of error-free signal                                                  a short failure or AIS (that lasts long                                       enough for it to be detected by the                                           part of the NE performing the STS Path                                        PM function*)                                                                1 second of error-free signal                                                 2 more B3 errors.                                                            The NE should accumulate 3 STS Path CVs and                                   ESs, 1 SES and 0 UASs.                                        If the state does not persist                                                                 With the standby OC-N line failed or locked                   long enough for the failure or                                                                out and the alarm delay set for 10 seconds,                   AIS condition to be declared,                                                                 send a signal containing the following:                       then the accumulation of the                                                                   1 B3 error                                                   other near end parameters is                                                                   1 second of error-free signal                                not inhibited.   5 seconds of failure or AIS - trouble                                         1 second of error-free signal                                                 2 more B3 errors.                                                            The NE should accumulate 3 STS Path CVs, 7                                    or 8 ESs, 5 or 6 SESs, and 0 UASs.                            If a protection switch re-                                                                    With the standby OC-N line available for                      stores the Path, then the                                                                     protection switching, send a signal con-                      accumulation of the other near                                                                taining the following:                                        end parameters is not inhibit-                                                                 1 B3 error                                                   ed even if the condition is                                                                    1 second of error-free signal                                declared.       20 seconds of Line failure or AIS.                                            The NE should perform a protection switch,                                    and accumulate 1 STS Path CV, 2 ESs, 1 SES,                                   and 0 UASs.                                                   __________________________________________________________________________     *See Failure Insertion Timing Issues.                                    

PM Accumulation During Unavailable Time

Tables X and XI list example tests that could be used to test an NEagainst the criteria related to the accumulation of PM parameters duringunavailable time. Similar to a number of the previous tests, these referspecifically to the accumulation of the STS Path parameters. In thiscase, the tests for the VT Path parameters would be very similar, and ifan NE provides a Line UAS parameter then these tests could also be usedat the Line layer without any change in the expected results. For theSection layer parameters (and the Line layer parameters if the NE doesnot provide a Line UAS parameter) the tests should still be performed,although the expected results would be different. Some SONET NEs thathave been analyzed have incorrectly inhibited the accumulation of Lineparameters in certain situations, even though a line UAS parameter wasnot provided.

                                      TABLE X                                     __________________________________________________________________________    SONET Performance Monitoring, Unavailable Time Caused By Failures And         AISs                                                                          Example Criteria       Test Method                                            __________________________________________________________________________    (R) The accumulation of STS Path                                                                     With the standby OC-N line failed or locked out                               and the alarm delay set for 2.5                        CVs, ESs, and SESs are inhibited                                                                     seconds, send a signal containing the following:       during unavailable time caused by a                                                                   1 B3 error                                            declared LOS, LOF, Line AIS, STS LOP,                                                                 1 second of error-free signal                         or STS AIS condition. STS Path UASs                                                                   5 seconds of failure or AIS                           are accumulated during that time.                                                                     1 second of error-free signal                         The time that the NE considers to be                                                                  2 more B3 errors.                                     unavailable time is consistent with                                                                  The NE should accumulate 3 STS Path CVs, 2 ESs, 0                             SESs and 5 or 6 UASs.                                  FIGS. 11 and 12.                                                              The time that the NE considers to be                                                                 With the standby OC-N line failed or locked out,                              the alarm delay set to 2.5                             unavailable time is consistent with                                                                  seconds, and the clear delay set to 15 seconds,                               send a signal containing the                           FIGS. 11 and 12 (trouble reappears                                                                   following:                                             during clear delay).    1 B3 error                                                                    1 second of error-free signal                                                10 seconds of failure or AIS                                                  12 seconds of error-free signal                                                1 second of failure or AIS                                                   12 seconds of error-free signal                                                1 second of failure or AIS                                                    1 second of error-free signal                                                 2 more B3 errors.                                                            The NE should accumulate 3 STS Path CVs, 2 ESs, 0                             SESs, and 36 or 37 UASs.                               The time that the NE considers to be                                                                 With the standby OC-N line failed or locked out,                              the alarm delay set to 2.5                             unavailable time is consistent with                                                                  seconds, and the clear delay set to 15 seconds,                               send a signal containing the                           FIGS. 11 and 12 (SESs occurring                                                                      following:                                             immediately before the trouble, and                                                                   1 B3 error                                            during the clear delay period).                                                                       1 second of error-free signal                                                 5 seconds with a burst of 10 B3 errors each                                  second                                                                        10 seconds of failure or AIS                                                   8 seconds of error-free signal                                                4 seconds with a burst of 10 B3 errors each                                  second                                                                         1 second of error-free signal                                                 2 more B3 errors.                                                            The NE should accumulate 3 STS Path CVs, 2 ESs, 0                             SESs, and 27 or 28 UASs.                               The time that the NE considers to be                                                                 With the standby OC-N line failed or locked out,                              the alarm delay set to 2.5                             unavailable time is consistent with                                                                  seconds, and the clear delay set to 5 seconds,                                send a signal containing the                           FIGS. 11 and 12 (even if the trou-                                                                   following:                                             ble clears, the unavailable time is                                                                   1 B3 error                                            not considered to be over until the                                                                   1 second of error-free signal                         start of 10 consecutive seconds that                                                                 10 seconds of failure or AIS                           are not SESs).          8 seconds of error-free signal                                                8 seconds with 10 B3 errors per second                                        2 seconds of error-free signal                                                8 seconds with 10 B3 errors per second                                        1 second of error-free signal                                                 2 more B3 errors.                                                            The NE should accumulate 3 STS Path CVs, 2 ESs, 0                             SESs, and 36 or 37 UASs.                               __________________________________________________________________________

                                      TABLE XI                                    __________________________________________________________________________    SONET Performance Monitoring,                                                 Unavailable Time Caused By Consecutive SESs                                   Example Criteria                                                                             Test Method                                                    __________________________________________________________________________    (R) The accumulation of STS                                                                  Send a signal containing the following:                        Path CVs, ESs, and SESs are                                                                   9 seconds with a burst of 10 B3 errors                        inhibited during unavailable                                                                   each second                                                  time caused by 10 or more                                                                     2 error-free seconds                                          consecutive SESs. STS Path                                                                    9 seconds with a burst of 10 B3 errors                        UASs are accumulated during                                                                    each second.                                                 that time.     The NE must accumulate 180 STS Path CVs, 18                                   ESs, 18 SES, and 0 UASs.                                       The time that the NE considers                                                               Send a signal containing the following:                        to be unavailable time is                                                                     1 B3 error                                                    consistent with FIGS. 11 and                                                                  1 second of error-free signal                                 12 (not considered unavailable                                                                6 seconds with a burst of 10 B3 errors                        time until the start of 10                                                                     each second                                                  consecutive seconds that are                                                                  2 error-free seconds                                          SESs).         10 seconds with a burst of 10 B3 errors                                         each second                                                                  1 second of error-free signal                                                 2 more B3 errors.                                                            The NE must accumulate 63 CVs, 8 ES, 6                                        SESs, and 10 UASs.                                             The time the NE considers to                                                                 Send a signal containing the following:                        be unavailable time is consis-                                                                1 B3 error                                                    tent with FIGS. 11 and 12                                                                     1 second of error-free signal                                 (unavailable time is not exit-                                                               10 seconds with a burst of 10 B3 errors                        ed until the start of 10 con-                                                                  each second                                                  secutive seconds that are not                                                                 8 error free seconds                                          SESs).          6 seconds with a burst of 10 B3 errors                                         each second                                                                  1 second of error-free signal                                                 2 more B3 errors.                                                            The NE must accumulate 3 CVs, 2 ES, 0 SESs,                                   and 24 UASs.                                                   __________________________________________________________________________

When a UAS parameter is provided at a particular layer, there are twomain types of incoming signals that should cause the NE to consider aperiod of time to be unavailable. The first of these is a declaredfailure or AIS condition that affects the line or path being monitored,and the second is a signal with 10 or more consecutive SESs at thatlayer. Both of these types of signals are covered in the examples, asare the criteria for entering and exiting unavailable time which areillustrated in FIGS. 11 and 12. It is important to remember that a Linefailure or AIS should only cause unavailable time at the Path layer ifit affects the path (i.e. it occurs on the active OC--N line), and thepath is not restored by a protection switch.

As in several of the tests described previously, in several of thesetests a single error is sent one second before the start of the troubleor string of consecutive SESs, and two more errors are sent one secondafter the end of the trouble. These errors are intended to test if theNE only inhibits the accumulation during the correct seconds. Theyshould be sent a full second before and after the trouble so that theycannot be detected as occurring in the same second as the trouble andthe NE will be required to accumulate them. In addition, for theseerrors to be apparent in the performance monitoring PM data, the errorsthat are intended to cause an SES should be inserted in a short burst.Otherwise, they may be detected as occurring in two separate 1-secondintervals and the first and last seconds in which they are inserted willnot contain enough errors to be counted as SESs. If this occurs, thenthe counts from those two seconds could mask the other counts.

Far End PM

In TR 253, the capability to accumulate several far end STS and VT PathPM parameters is required for Path terminating NEs. These parameters arevery similar to the near end Path parameters, and are based on the STSand VT Path Far End Block Error (FEBE) codes and Yellow signals receivedfrom the far end Path terminating NE. In general, to test an NE's farend PM, a SONET test set needs to be capable of sending FEBE codes atvarious rates and Yellow signals for various time periods, as specifiedby the user.

NDF Results

When an NE that terminates an STS or VT receives a set NDF, it must usethe pointer value accompanying that NDF to interpret the incoming SPEand extract the payload. The NE is required to start using the newpointer value at the first occurrence of the offset indicated by thatpointer, and tests can be designed to determine if the NE meets thatrequirement. However, the interpretation of the results of those testsmay be dependent on the NE's internal architecture. Given below is adescription of test signals that can be used in the NDF tests, andseveral items that should be considered when the test results areinterpreted.

NDF Test Signals

Referring now to FIG. 16, there is disclosed a method that an be used tomeasure the time an NE takes to begin using a new pointer value. Themethod is to send a signal containing one pointer value to the NE,change to a different pointer value (with the NDF set in the firstpointer word containing the NDF), and monitor the DSn extracted anddropped by the NE from the affected SPE. Both before and after thepointer change, the DSn information bits contained in the SPE would needto contain a pattern that can be easily distinguished from "garbage"data and AIS when the DSn signal is dropped by the NE (e.g. a "1100"pattern), while the other bytes in the SPE, such as the Path overheadbytes, would need to contain a different pattern (e.g. all zeros). Also,the two pointer values would need to be chosen so that the bytelocations within the SONET frame that contain the Path overhead byteswith the new pointer value are locations that contained information bitswith the old pointer value. After the NDF and new pointer value aresent, if the NE continues to use the old pointer value for some timeafter the first occurrence of the new offset, then the (new) overheadbytes will be read as information bits. Since the overhead bytes containa different pattern than the information bits, those bytes will beapparent in the dropped DSn. The delay between the first occurrence ofthe new offset and the time when the dropped DSn signal consists of onlythe pattern contained in the information bytes of the SPE indicates howlong the NE delays using the new pointer value. Depending on the testsignals and the characteristics of the NE under test, it may benecessary to capture the dropped DSn signal in some form for lateranalysis. It may be possible to capture the DSn signal directly, or thatsignal may be looped back to the low speed input of the NE so that itwill be mapped and multiplexed into a SONET signal which can be capturedby the SONET test set.

FIG. 17 gives an example of a signal that could be used in a VT NDFtest. It also shows the DS1 signals that would be generated if the NEmet the requirement on using the new pointer value at the firstoccurrence of the new offset, and the results if it delayed the use forone superframe. Note that in this example the VT pointer is beingchanged from one value to a larger value. For this type of change, thereare no requirements concerning the NE's use of the bytes between the oldoffset and the new offset, so those bytes (which are shown set toall-ones in the example) may or may not appear in the dropped DS1signal.

NDF Test Result Interpretation

Most of the SONETADMs that had been analyzed could be used to transportDS3 payloads in STS-1 SPEs and/or DS1 payloads in VT1.5 SPEs. For thesetypes of payloads, it would be logical to perform STS NDF tests for bothDS1 and DS3 payloads, and VT NDF tests for DS1 payloads. However, thecase of an STS NDF with a DS1 payload is much more complicated than theother two cases, and satisfactory tests and guidelines for interpretingtest results have not been developed. When an NE that is dropping DS1payloads receives an STS NDF, it is possible that those payloads will beinterrupted for some time while the NE determines the phase of the VTsuperframe contained in the shifted STS SPE (using the new H4 bytesequence). Also, it may take the NE one or more superframes to determineif the VT pointers embedded in the shifted STS SPE contain the sameoffsets as they did before the NDF, and to determine the new offsets ifthey have changed. Since the criteria in TR 253 does not specificallyaddress these issues, any tests that are developed would primarily beused for information purposes.

In the case of an STS NDF with a DS3 payload, or a VT NDF with a DS1payload, the primary factor that complicates the analysis of the resultsis the internal architecture of the NE. If the NE only processes the STSor VT pointer at the time that it terminates the STS or VT SPE, then thedropped DSn payload will directly reflect any delay that the NE has inusing the new pointer value, and the interpretation of the results isreasonably straight forward. However, if the NE has one of several otherpossible architectures, the incoming pointer will not be directly usedby the circuit pack that extracts the DSn payload, and that could causea delay even though the NE may conform with the requirements.

In an NE using one certain architecture, the incoming pointer would beprocessed on one circuit pack, which would then generate a new pointervalue for use in the signal it sends to a lower speed circuit pack. Thelower speed circuit pack would then have to process the new pointer(i.e. the internally generated pointer) in order to terminate the SPEand extract the DSn payload. In this architecture, the higher speedcircuit pack is simply processing the STS (VT) pointer and then passingthe STS (VT), which is equivalent to an ADM that processes the pointerson through STSs (VTs). When such an ADM receives an NDF on a through STS(VT), it should send an NDF downstream in the next available pointer.For both an ADM and a high speed pack that performs pointer processing,the frame rate of the incoming signal is controlled by the far end NE'sclock and the frame rate of the outgoing signal is controlled by thelocal NE's clock, which could result in an offset in both time andfrequency between the incoming and outgoing pointers. The offset in timecould cause up to a one frame (superframe) delay between the time thatthe NE receives the incoming NDF and time that the next pointer isavailable to relay that pointer change information on downstream (i.e.to insert an NDF downstream). If there is an offset in frequency, thenthe offset in time will change, and that should cause the delays in theuse of the incoming NDF tests to vary. Thus, if one NDF test indicatesthat the NE delays the use of the incoming NDF but that the delay isless than one frame (superframe), then the frequencies of the timingreferences used to time the test set and the NE should be temporarilyoffset to determine if the measured delay changes. If the delay doeschange, then the NE may still conform with the requirements.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

What is claimed is:
 1. A method for testing a SONET network elementhaving performance monitoring parameters defined for at least onelogical layer and capable of performing at least one performancemonitoring function, the network element having a normal operating stateand a trouble state, the method comprising the steps of:generating abasic test signal; modifying the basic test signal to obtain a modifiedtest signal; detecting the modified test signal at the network elementto cause the network element to change from the normal operating stateto the trouble state, the network element initiating a set of actions inresponse to the modified test signal; monitoring the modified testsignal to determine whether the network element is to change back fromthe trouble state to the normal operating state, wherein the improvementcomprises: the modified test signal includes a first error portion whichis followed by a first error-free portion having a first predeterminedtime period followed by a trouble portion of sufficient length to bedetected and cause the network element to change from the normaloperating state to the trouble state, said trouble portion beingfollowed by a second error-free portion having a second predeterminedtime period and followed by a second error portion, wherein the modifiedtest signal tests the at least one logical layer and wherein the set ofactions includes inhibiting accumulation of the parameters during a timeperiod in which the network element is in the trouble state andaccumulating parameters associated with the first and second errorportions of the modified test signal.
 2. The method as claimed in claim1 wherein the set of actions includes a subset of immediate actions anda subset of delayed actions.
 3. The method as claimed in claim 1 whereinthe first error portion includes a single error and the second errorportion includes two errors.
 4. The method as claimed in claim 3 whereinthe single error of the first error portion is identical to each of thetwo errors of the second error portion.
 5. The method as claimed inclaim 1 wherein the first predetermined time period is equal to thesecond predetermined time period.
 6. The method as claimed in claim 5wherein the first and second predetermined time periods are both equalto one second.
 7. A system for testing a SONET network element havingperformance monitoring parameters defined for at least one logical layerand capable of performing at least one performance monitoring function,the network element having a normal operating state and a trouble state,the system comprising:means for generating a basic test signal; meansfor modifying the basic test signal to obtain a modified test signal;means for detecting the modified test signal at the network element tocause the network element to change from the normal operating state tothe trouble state, the network element initiating a set of actions inresponse to the modified test signal; means for monitoring the modifiedtest signal to determine whether the network element is to change backfrom the trouble state to the normal operating state, wherein theimprovement comprises: the modified test signal includes a first errorportion which is followed by a first error-free portion having a firstpredetermined time period which is followed by a trouble portion ofsufficient length to be detected and cause the network element to changefrom the normal operating state to the trouble state, said troubleportion being followed by a second error-free portion having a secondpredetermined time period which is followed by a second error portion,wherein the modified test signal tests the at least one logical layerand wherein the set of actions includes inhibiting accumulation of theparameters during a time period in which the network element is in thetrouble state and accumulating parameters associated with the first andsecond error portions of the modified test signal.
 8. The system asclaimed in claim 7 wherein the set of actions includes a subset ofimmediate actions and a subset of delayed actions.
 9. The system asclaimed in claim 7 wherein the first error portion includes a singleerror and the second error portion includes two errors.
 10. The systemas claimed in claim 7 wherein the single error of the first errorportion is identical to each of the two errors of the second errorportion.
 11. The system as claimed in claim 7 wherein the firstpredetermined time period is equal to the second predetermined timeperiod.
 12. The system as claimed in claim 11 wherein the first andsecond predetermined time periods are both equal to one second.
 13. Amethod for testing a SONET network element, the method comprising thesteps of:generating a test signal including a payload pointer initiallyhaving a first pointer value and subsequently having a second pointervalue different from the first pointer value, the payload pointerindicating the location of the beginning of an incoming synchronouspayload envelope (SPE) of the test signal in order to extract a payloadsignal therefrom, the payload signal including information bytes havinga predetermined pattern; detecting the test signal at the networkelement so that the network element extracts the payload signal from theSPE and to detect the information bytes having the predeterminedpattern; and determining a delay between a first occurrence of thesecond pointer value and a time when the extracted payload signalincludes only the information bytes having the predetermined pattern,wherein the delay indicates how long the network element delays in usingthe second pointer value.
 14. A system for testing a SONET networkelement, the system comprising:means for generating a test signalincluding a payload pointer initially having a first pointer value andsubsequently having a second pointer value different from the firstpointer value, the payload pointer indicating the location of thebeginning of an incoming synchronous payload envelope (SPE) of the testsignal in order to extract a payload signal therefrom, the payloadsignal including information bytes having a predetermined pattern; meansfor detecting the test signal at the network element so that the networkelement extracts the payload signal from the SPE and to detect theinformation bytes having the predetermined pattern; and means fordetermining a delay between a first occurrence of the second pointervalue and a time when the extracted payload signal includes only theinformation bytes having the predetermined pattern, wherein the delayindicates how long the network element delays in using the secondpointer value.